Microlenses with diameters ranging from 25 $\mu$m to 100 $\mu$m are made by melting "islands" of photoresist on a glass substrate. This method is simple and easily accomplished using existing photolithography techniques. The resulting microlenses show excellent surface and cross section profiles obtained by scanning electron microscope and stylus measurement respectively. In order to evaluate the optical properties of the lenses, focal lengths are measured and focal spot images are taken by a frame grabber. In the case of the lens with diameter of 100 $\mu$m, the focal length is 165 $\mu$m and its spot size is 4.5 $\mu$m. This spot size is a little larger than the diffraction-limited spot size of 2.5 $\mu$m which is attributed to the degradation of the image caused by the relaying optics. The exact measurement of the optical properties require interferometric methods. Phase-Shifting Interferometry let us know what the wave-front is like after plane wave has passed the microlenses. The wave-front is analysed by the classical wave propagation theory thereby the the focal spot image and the frequency response of the lenses are obtained. Especially the frequency response of the manufactured microlenses approaches that of the diffraction-limited one. In summary, the microlenses made by photoresist melting method have advantages in the easiness of their fabrication, the excellent quality of its surface profile and the optical property nearly approaching that of the diffraction limited lens.
